Novel Electrodes

ABSTRACT

The present disclosure provides electrodes that possess components capable of indicating to an end-user when the electrode is in need of replacement. The electrode includes a substrate and a conductive composition on at least a portion of a surface of the substrate. In embodiments, the conductive composition includes at least one hydrogel and at least one component that precipitates from the hydrogel after repeated use of the electrode, thereby providing an indication to replace the electrode. In other embodiments, the electrodes include a hydrogel in combination with a pH indicator which changes its color or opacity upon repeated use of the electrode, thereby indicating an appropriate time for changing or replacing the electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/261,134, filed Oct. 30, 2008, now U.S. Pat. No. ______. Thisapplication is also a continuation-in-part of U.S. patent applicationSer. No. 12/261,122, filed Oct. 30, 2008, now U.S. Pat. No. ______. BothU.S. patent application Ser. Nos. 12/261,134 and 12/161,122 claim thebenefit of and priority to U.S. Provisional Patent Application No.60/984,869, filed Nov. 2, 2007. The entire disclosures of each of theforegoing applications and/or patents are incorporated by referenceherein.

TECHNICAL FIELD

The present disclosure relates to hydrogels suitable for use asconductive compositions, methods of making these compositions, and theuse of these compositions with medical electrodes.

BACKGROUND OF RELATED ART

Hydrogels constitute a broad class of materials which may be completelywater soluble or swell extensively in water but are not completely watersoluble. They have been used in a variety of biomedical applications andmay be applied in bulk forms which vary from clear to opaque, and from arelatively stiff to a relatively soft consistency. Sometimes the bulkforms are reinforced by reinforcement members which may be woven ornon-woven fabrics to increase the composite strength and/or dimensionalstability. Hydrogels have also been used as coatings for variousbiomedical applications.

Medical electrodes are used to transmit electrical signals or currentsbetween the body of a patient and external medical equipment. Theseelectrodes may include a conductive composition adhered to or otherwisein contact with, the skin of the patient, and a conductor, which iselectrically connected to the conductive composition and to the externalmedical equipment.

Hydrogels for use as conductive compositions with medical electrodesremain desirable.

SUMMARY

The present disclosure provides electrodes that possess componentscapable of indicating to an end-user when the electrode is in need ofreplacement. In embodiments, an electrode of the present disclosure mayinclude a substrate and a conductive composition on at least a portionof a surface of the substrate, the conductive composition including atleast one hydrogel and at least one component having a limitedsolubility in water that precipitates from the hydrogel after repeateduse of the electrode thereby providing an indication to replace theelectrode.

In other embodiments, an electrode of the present disclosure may includea substrate and a conductive composition on at least a portion of asurface of the substrate, the conductive composition including at leastone hydrogel and at least one thermochromic material which will changeits color or opacity on exposure to heat thereby providing an indicationto replace the electrode.

In other embodiments, an electrode of the present disclosure may includea substrate, and a conductive composition on at least a portion of asurface of the substrate, the conductive composition including at leastone hydrogel and at least one pH indicator component which will changeits color or opacity on exposure to a change in pH, thereby providing anindication to replace the electrode.

In other embodiments, an electrode of the present disclosure may includea substrate, and a conductive composition on at least a portion of asurface of the substrate, the conductive composition including at leastone hydrogel and at least one pH indicator component that will changeits color or opacity on exposure to a specified pH, in some embodimentsof from about 2 to about 4, in other embodiments from about 8 to about10, thereby providing an indication to replace the electrode.

In yet other embodiments, an electrode of the present disclosure mayinclude a substrate and a conductive composition on at least a portionof a surface of the substrate, the conductive composition including atleast one hydrogel and at least one ionic component that may beiontophoretically delivered from the hydrogel after repeated use of theelectrode thereby providing an indication to replace the electrode.

In yet other embodiments, an electrode of the present disclosure mayinclude a substrate, a conductive composition on at least a portion of asurface of the substrate, the conductive composition including at leastone hydrogel, and a heat sensitive component on a portion of a surfaceof the substrate opposite the surface of the substrate having theconductive composition that will provide an indication to replace theelectrode on exposure to heat.

Methods for producing electrodes and the components thereof are alsoprovided, as are methods for their use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a medical electrode including theconductive composition of the present disclosure.

FIG. 2 is a cross-sectional view of the medical electrode of FIG. 1.

FIG. 3 is a cross-sectional view of a snap medical electrode.

FIG. 4 is a cross-sectional view of an alternate medical electrode ofthe present disclosure.

FIG. 5 is a top view of an electrode of the present disclosure prior touse.

FIG. 6 is a top view of an electrode of the present disclosure afteruse.

DETAILED DESCRIPTION

Any adhesive application, including those involving tissue, are withinthe purview of the hydrogel compositions of the present disclosure. Inembodiments, hydrogels may be utilized as adhesives and/or conductivecompositions for medical electrodes. The hydrogels of the presentdisclosure may include components that provide an indication to anend-user that the hydrogel is reaching the end of its useful life and,thus, the electrode should be replaced.

As used herein, the term “hydrogel” may refer to a wide variety ofpolymer-based compositions. These materials may be synthesized forexample from monomer(s) or from monomer(s) mixed with polymer(s) orcross-linked polymer solutions in water. They may be obtained bychemical modification of existing polymer(s) or by adding water toexisting dry polymers.

Any biocompatible hydrogel may be utilized in accordance with thepresent disclosure. Generally speaking, a hydrogel according to thepresent disclosure may include a coherent, three-dimensional aqueouspolymer system capable of imbibing water without liquefying. Inembodiments, insolubility in water may be provided by crosslinking thehydrogel polymer. In embodiments, hydrogels or water-containing gels ofthe present disclosure may include water and various chemical substancesincluding gelatin; polysaccharides; crosslinked acrylamide polymers,hydroxyethylmethacrylate polymers; crosslinked polyhydroxyethylacrylate;polymerized, crosslinked 2-acrylamido-2-methylpropane sulfonic acidpolymers or one of their salts such as the sodium or potassium type;crosslinked polyvinylpyrrolidone; polyacrylic acid; copolymers of theaforementioned monomers with each other, copolymers of theaforementioned monomers with other polymers such as polystyrene or othernon-hydrogel-forming polymers, one or more salts of the foregoing, andcombinations thereof.

For example, by cross-linking homopolymers of an acrylamide derivativesuch as 2-acrylamido-2-methylpropanesulfonic acid or one of its salts,hydrogels may be formed. Copolymers thereof may also be formed in thesame way with acrylamide. Cross-linked homopolymers of acrylic acid andof methacrylic acid, their salts and copolymers thereof do likewise, asdo other acrylic cross-linked homopolymers and copolymers.

Hydrogels of the present disclosure derive their adhesive properties inpart from their ability to absorb water. When a relatively dry body ofhydrogel contacts moisture, such as the moisture in tissue, particularlyinternal tissue, or any other moist surface, it develops an aggressiveadhesive nature. When the polymer of the hydrogel is crosslinked to anadequate degree, the bulk hydrogel is strong enough, even when swelledwith additional liquid, to provide adhesive support for pacing leads,thereby establishing extended connection of the lead to tissue.

Excessive crosslinking decreases the tack of the hydrogel. Too littlecrosslinking decreases its cohesive strength. Thus, in embodiments, acrosslinking agent may be utilized in forming the polymer suitable as ahydrogel of the present disclosure.

In use, a hydrogel of the present disclosure may contain the polymer orcopolymer, and any other additives, including components utilized toform the copolymer, in an amount from about 20% by weight to about 97%by weight of the hydrogel, with the balance being water and/or ahumectant.

In some embodiments, a suitable hydrogel for use as a conductivecomposition may include a copolymer. Non-limiting examples of suitablecopolymers may include a first monomer, such as a mixture of acrylicacid and a salt thereof, and a second monomer, such as one of moremonomers selected from CH₂═CHC(O)XR, in which X is O or NH and R is anunsubstituted or substituted alkyl group of 1 to 5 carbon atoms. Thehydrogel may also include water; an electrolyte or mixture ofelectrolytes; a polymerization initiator; neutralizer a such as sodiumhydroxide; optionally, a crosslinking agent; and optionally, athickener.

In embodiments, a first monomer which may be used to form a copolymerfor use in a hydrogel includes acrylic acid, a salt thereof, or amixture thereof. The copolymer thus produced by polymerization includesacid acrylate moieties (—CO₂H and/or —CO₂M, in which M is a cation suchas sodium ion, potassium ion, lithium ion, ammonium or substitutedammonium ion, etc.) directly attached to the polymer backbone.

In embodiments, a copolymer utilized in a hydrogel of the presentdisclosure may include a second monomer which may be one of moremonomers selected from CH₂═CHC(O)XR, in which X is O or NH and R is anunsubstituted or substituted alkyl group of 1 to 5 carbon atoms. Thepolymer produced by this polymerization includes groups of thestructure—C(O)XR directly attached to the polymer backbone.

Suitable unsubstituted alkyl groups are methyl, ethyl, n-propyl,n-butyl, and n-pentyl. Suitable substituents that may be present in asubstituted alkyl group are halo (such as F, Cl, or Br) cyano,carboxylic acid and salts thereof (i.e., —CO₂H or —CO₂M, in which M is acation), phosphate and salts thereof, and sulfonic acid and saltsthereof. An example of such a substituted alkyl group is(3-sulfopropyl)acrylic acid ester, potassium salt. Suitable secondmonomers include 2-acrylamido-2-methylpropane sulfonic acid(CH₂═CH—CONHC(CH₃)₂—CH₂—SO₃H) and/or a salt thereof. Suitable saltsinclude the sodium, lithium, potassium, ammonium, and substitutedammonium salts, and mixtures thereof.

In embodiments, the second monomer utilized in a copolymer component ofa hydrogel of the present disclosure is 2-acrylamido-2-methylpropanesulfonic acid sodium salt (NaAMPS) (CH₂═CH—CONHC(CH₃)₂—CH₂—SO₃ ⁻M⁺).Thus, in some embodiments, the first monomer utilized in a copolymercomponent of a hydrogel of the present disclosure may include a mixtureof acrylic acid and sodium acrylate, and the second monomer may includesodium 2-acrylamido-2-methylpropane sulfonate.

The first monomer (acrylic acid and/or salt or salt thereof, calculatedas acrylic acid) may be present in an amount of from about 8 wt % toabout 85 wt % of copolymer in the hydrogel, in embodiments from about 10wt % to about 80 wt %, of the total amount of the copolymer in thehydrogel. The second monomer, in embodiments NaAMPS, may be present inan amount of from about 15 wt % to about 92 wt % of the copolymer in thehydrogel, in embodiments from about 20 wt % to about 90 wt % of thecopolymer in the hydrogel.

Optionally, an effective amount of a cross-linking agent or mixture ofcross-linking agents may be utilized to form the copolymer component ofa hydrogel of the present disclosure. An effective amount ofcross-linking agent is an amount that produces a conductive compositionwith the desired physical properties, such as coherence and adhesion,and electrical properties. Although the amount required will depend on,for example, the molecular weight of the cross-linking agent, the numberof ethylenically unsaturated, free radical polymerizable groups presentin the cross-linking agent, the amount of free radical polymerizablemonomers present in the monomer mix, when the cross-linking agent ispresent, the amount of crosslinking agent will be present in an amountof from about 0.01 wt % to 1 wt % of the copolymer utilized in thehydrogel, in embodiments from about 0.02 wt % to 0.08 wt % of thecopolymer utilized in the hydrogel.

Suitable cross-linking agents include free radical polymerizablemonomers that possess more than one ethylenically unsaturated, freeradical polymerizable group. Numerous crosslinking agents polymerizableby free-radical initiated polymerization are within the purview of thoseskilled in the art. Crosslinking agents include, for example,bis-acrylamides and methacrylamides, such as N,N′-methylenebis-acrylamide; acrylate and methacrylate esters of polyols, such as,ethylene glycol diacrylate and dimethacrylate, diethylene glycoldiacrylate and dimethacrylate, trimethylolpropane triacrylate andtrimethacrylate, ethoxylated trimethylolpropane triacrylate andtrimethacrylate; pentaerythritol triacrylate and trimethacrylate,pentaerythritol tetraacrylate and tetramethacrylate, and polyethyleneglycol diacrylates and dimethacrylates, such as the diacrylates anddimethacrylates of polyethylene glycols having a molecular weight offrom about 200 to about 600. In embodiments, a suitable crosslinkingagent may include N,N′-methylene bis-acrylamide [(CH₂═CHCONH)₂CH₂].

In embodiments, a polymerization initiator may be utilized with thefirst monomer and second monomer to form a copolymer for use in ahydrogel of the present disclosure. An effective amount of apolymerization initiator may be combined with the monomers to form sucha copolymer. As used herein, an effective amount is an amount thatproduces efficient polymerization of the monomers under polymerizationconditions to produce a hydrogel suitable for use as a conductivecomposition. Numerous free radical polymerization initiators are withinthe purview of those skilled in the art. The polymerization initiatormay be a single compound or a mixture of compounds. Thermal and/or photofree radical polymerization initiators, for example, may be used.

Suitable thermal free radical polymerization initiators include azocompounds, such as 2,2-azobisisobutyronitrile (AIBN). Suitable photofree radical polymerization initiators are disclosed in “Photoinitiatorsfor Free-Radical-Initiated Photoimaging Systems,” by B. M. Monroe and G.C. Weed, Chem. Rev., 93, 435-448 (1993) and in “Free RadicalPolymerization” by K. K. Dietliker, in Chemistry and Technology of UVand EB Formulation for Coatings, Inks, and Paints, P. K. T. Oldring,ed., SITA Technology Ltd., London, 1991, Vol. 3, pp. 59-525. Suitablefree radical photo polymerization initiators include, for example,1-hydroxycyclohexylphenyl ketone (HCPK, IRGACURE® 184);2-hydroxy-2-methyl-1-phenylpropan-1-one (DAROCUR® 1173);2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propan-1-one(IRGACURE® 2959), 2,2-dimethoxy-2-phenylacetophenone (benzildimethylketal, BDK, IRGACURE®651), benzophenone, a mixture of 50 wt %benzophenone and 50 wt % of 1-hydroxycyclohexylphenyl ketone (IRGACURE®500), and combinations thereof.

The polymerization initiator may be present in a copolymer utilized in ahydrogel in an amount less than about 1 wt % of the copolymer, inembodiments less than about 0.7 wt % of the copolymer, in otherembodiments less than about 0.4 wt % of the copolymer.

The hydrogel of the present disclosure may also include an electrolyteor a mixture of electrolytes. The electrolyte may be a salt, such aslithium chloride, sodium chloride, potassium chloride, magnesiumacetate, ammonium acetate, or a mixture thereof. In embodiments, asuitable electrolyte may include potassium chloride. The hydrogel maypossess the electrolyte in an amount from about 0.5 wt % to about 10 wt% of the hydrogel, in embodiments from about 1 wt % to about 8 wt % ofthe hydrogel.

The hydrogel utilized as a conductive composition may also include aneutralizer. Bases such as hydroxides, amines, Lewis bases, and mixturesthereof may be used as neutralizers. Non-limiting examples ofneutralizers include ammonium hydroxide, sodium hydroxide, potassiumhydroxide, lithium hydroxide, combinations thereof, and the like. If theacrylic acid and/or the second monomer, such as the2-acrylamido-2-methylpropane sulfonic acid, are included as monomers informing a copolymer for use in the hydrogel, it may be desirable to addneutralizer to neutralize some of the acid so that the pH of the mixtureis from about 3 to about 6.5.

Where utilized, a neutralizer may be present in an amount from about 2wt % to 8 wt % of the hydrogel.

In addition to a free radical initiator, small amounts of free radicalpolymerization inhibitors may be present with one or more of themonomers, and/or the crosslinking agent, and/or may be added to themixture to prevent premature polymerization of the reaction mixture.Suitable free radical polymerization inhibitors include, for example,hydroquinone, 4-methoxyphenol, di-t-butyl-p-cresol, pyrogallol, t-butylcatechol, benzoquinone, 4,4′-thio-bis-(3-methyl-6-t-butylphenol), and2,2′-methylene-bis-(4-methyl-6-t-butylphenol). When present, the amountof the polymerization inhibitor may be from about 0.01 wt % to about 5wt % of the hydrogel.

In some embodiments a thickener may be added to the hydrogel. Suitablethickeners include rheological modifiers which permit tailoring theviscosity of the hydrogel to permit its use as a conductive compositionwith a medical electrode. Non-limiting examples of such thickenersinclude silica, gums including xanthan gum, polymers including polyvinylpyrrolidone (PVP), polyacrylamides, polyacrylic acid (including thosesold under the name CARBOPOL), salts thereof, combinations thereof, andthe like. Where utilized, a thickener may be present in a hydrogel ofthe present disclosure in an amount from about 0.1 wt % to about 8 wt %of the hydrogel, in embodiments from about 0.5 wt % to about 5 wt % ofthe hydrogel.

Other conventional ingredients of conductive compositions may be presentin the hydrogel. For example, humectants and medicinal agents, includingantimicrobials, antiseptics, analgesics, disinfectants, and the like,may be added to a hydrogel.

Water is present in the mixture. The amount of water includes any waterpresent in any of the ingredients and any water added with ingredientsthat are in water solution, such as the monomers, the crosslinkingagent, the neutralizer, etc. In embodiments, humectants may be added tothe water phase of a hydrogel utilized as a conductive composition in anelectrode of the present disclosure. Humectants which may be usedinclude non-volatile, non-toxic, water soluble or water miscible viscousliquids at room temperature. Suitable humectants include, but are notlimited to, polyhydric alcohols such as glycerol, sorbitol, ethyleneglycol, propylene glycol, polyethylene glycols (PEG) of varyingmolecular weights including PEG 300, PEG 400 and PEG 600, polypropyleneglycols, combinations thereof, and the like. The humectant may beutilized in combination with water or without water. Where utilized withwater, the ratio of water to humectant may be from about 1:10 to about10:1.

As noted above, in use, a hydrogel of the present disclosure may containthe polymer or copolymer and any other additives described herein in anamount from about 20% by weight to about 97% by weight, with the balancebeing water and/or a humectant in an amount from about 3% to about 80%by weight of the hydrogel.

An example of a suitable polymer which may be utilized as the hydrogelincludes RG-63B, commercially available from Covidien.

The monomers and any additional components described above may be mixedand spread or coated as a layer on a release liner, for example asiliconized release substrate such as silicone coated polyethyleneterephthalate film, or other substrate prior to polymerization. Freeradical polymerization may be initiated by, for example, heating themixture when a thermal free radical polymerization initiator is presentin the mixture, or exposing the mixture to actinic radiation when aphotoinitiated free radical polymerization initiator is present in themixture. Any convenient source or sources of actinic radiation providingwavelengths in the region of the spectrum that overlap the absorptionbands of the photoinitiated free radical polymerization initiator can beused to activate polymerization. The radiation can be natural orartificial, monochromatic or polychromatic, incoherent or coherent, andfor high efficiency should correspond closely in wavelengths to theabsorption bands of the polymerization initiator. Conventional lightsources include fluorescent lamps, mercury vapor lamps, metal additivelamps, and arc lamps. Useful lasers are those whose emissions fallwithin or overlap the absorption bands of the photoinitiated freeradical polymerization initiator. Although, if desired, the mixture maybe degassed before polymerization and/or the polymerization may becarried out under an inert atmosphere, it is not necessary to degas themixture before polymerization or to carry out the polymerization underan inert atmosphere.

Following polymerization, the resulting conductive composition maytransferred to a conductive substrate. Alternatively, the conductivecomposition may be adhered to a conductive substrate, and the releaseliner left in place to protect the conductive composition until it isready for use.

Change Indicating Component

In accordance with the present disclosure, an electrode of the presentdisclosure also possesses a component which may be utilized to indicatethe useful life of the hydrogel has expired or is about to expire. Thisindication will identify for an end-user that the electrode in useshould be replaced and a new electrode should be utilized. In someembodiments, the component indicating that it is time to change orreplace the electrode may be included in the hydrogel described above aspart of a conductive composition. In other embodiments, the componentindicating that it is time to change or replace the electrode may be aseparate component applied to an electrode that is not included in thehydrogel described above.

In embodiments, a pH indicator may be added to the hydrogel or utilizedseparately to indicate it is time to change or replace the electrode.The pH indicator component may change its color or opacity on exposureto a change in pH, thereby providing an indication to replace theelectrode. For example, in some embodiments, as the electrode isutilized, hydrolysis of the hydrogel occurs. Hydrolysis of water in thehydrogel may increase the concentration of hydrogen ions in the hydrogelon the anode side, thereby reducing the pH of the hydrogel. The hydroxylion increase in concentration on the cathode side will increase the pHof the hydrogel. The pH indicator may be selected so that it develops acolor or opacity, or changes color, as the pH drops or increasesdepending on whether one looks at the anode side or cathode side.

A pH indicator may provide two reasons to indicate the end of the usablelife of an electrode: water loss increases the impedance of theelectrode, which in turn generates heat, so changing an electrode inresponse to the pH indicator may avoid thermal burns; and the drop in pHindicates the generation of acid due to hydrolysis, or an increase in pHindicates the production of hydrogel ions, so changing an electrode inresponse to the pH indicator may avoid chemical burns.

Suitable pH indicators which may be utilized include, but are notlimited to, brilliant green, bromophenol blue, m-cresol purple, m-cresolred, eosin Y, methyl orange, methyl violet, combinations thereof, andthe like. The pH indicator may be present in a hydrogel in a suitableamount of from about 50 ppm to about 1000 ppm, in embodiments from about100 ppm to about 900 ppm.

Suitable pH indicators are commercially available from suppliers such asAldrich Chemicals, Riedel-deHaen, Mallinckrodt Chemicals, SpectrumChemicals, and the like. Listed below are details regarding some of theabove pH indicators, as well as others, which may be utilized inaccordance with the present disclosure.

Brilliant green (C₂₇H₃₄N₂O₄S), as supplied, is about 90% activecrystals, and may be used as about a 1% solution in water. Brilliantgreen is yellow at a pH of about 0 to about 2.5, and green at a pH ofabout 2.6 and higher.

Bromophenol blue (C₁₉H₁₀Br₄O₅), as supplied, is about 0.04% activesolution in water, and the same in use. Bromophenol blue is yellow at apH of below about 3, and purple at a pH of about 4.6 and above.

m-Cresol purple (C₂₁H₁₈O₅S), as supplied, is about 90% active crystals,and may be used as about 0.04% solution in water. m-Cresol purple isyellow at a pH of about 1.2 and lower, and purple at a pH of about 2.8and higher.

m-Cresol red (C₂₁H₁₈O₅S), as supplied, is about 0.04% active solution inwater, and the same in use. m-Cresol red is orange at a pH of about 2and lower, and yellow at a pH of about 3 and higher.

Eosin Y (C₂₀H₈Br₄O₅), as supplied, is about 5% active solution in water,and about the same in use. Eosin Y is orange at a pH of about 0, andyellow at a pH of about 3 and higher.

Methyl orange (C₁₄H₁₄N₃NaO₃S), as supplied, is about 0.1% activesolution in water, and the same in use. Methyl orange is orange at a pHof about 3.1 and below, and yellow at a pH of about 4.4 and higher.

Methyl violet (C₂₄H₂₈N₃Cl), as supplied, is about 75% active crystals,and may be used as about 0.02% active solution in water. Methyl violetis yellow at a pH of about 0, and violet at a pH of about 1.6 andhigher.

Phenolphthalein (C₂₀H₁₄O₄), as supplied, is about 1% active solution inalcohol, and the same in use. Phenolphthalein is colorless at a pH ofbelow about 8, and fuchsia at a pH of above about 8.

Bromocresol green can be utilized at a concentration of about 0.1% inwater. Bromocresol green is yellow at a pH of about 3.8 and lower, andblue at a pH of about 5.4 and higher.

Bromocresol green-methyl red is pink at a pH of about 4.6 and lower, andgreen at a pH of about 5.6 and higher.

Bromocresol purple can be utilized at a concentration of about 0.1% inwater. Bromocresol purple is yellow at a pH of about 5.3 and lower, andpurple at a pH of about 6.8 and higher.

Bromothymol blue can be utilized at a concentration of about 1% inwater. Bromothymol blue is yellow at a pH of about 6 and lower, and blueat a pH of about 7.6 and higher.

Congo red can be utilized at a concentration of about 0.1% in solution.Congo red is blue at a pH of about 3 and lower, and red at a pH of about5 and higher.

Methyl red can be utilized at a concentration of about 0.1% in water.Methyl red is pink at a pH of about 4.2 and lower, and yellow at a pH ofabout 6.2 and higher.

Phenol red can be utilized at a concentration of about 0.04% in water.Phenol red is yellow at a pH of about 6.8 and lower, and red at a pH ofabout 8.2 and higher.

Thymol blue can be utilized at a concentration of about 0.4% in water.Thymol blue is red at a pH of about 1.2 and lower, yellow at a pH ofabout 2.8 to about 8, and blue at a pH of about 9.2 and higher.

As noted above, in embodiments combinations of pH indicators may beused.

In some embodiments, a pH indicator may be selected which will undergo achange in its appearance, for example change color or develop its coloror opacity, at a pH of from about 2 to about 4, in embodiments fromabout 2.25 to about 3.25. In other embodiments, the dye utilized maydevelop a color, change color, or become opaque at a pH of from about 8to about 10, as the electrode is utilized as described above.

In other embodiments, a salt or similar component having a limitedsolubility in water may be added to the hydrogel as a change indicatingcomponent. Enough of this material is added to the hydrogel so that itall dissolves, but is very near its saturation point. This material maybe opaque or possess a color. While in solution in the hydrogel, thecolor or opacity of the component is not observed. However, as theelectrode possessing a hydrogel of the present disclosure is utilizedand exposed to the atmosphere, water is lost. As more water is lost withrepeated use, the concentration of the component in the remaining waterof the hydrogel will reach the point of saturation, thereby becominginsoluble and precipitating out of the remaining hydrogel. The componentwill thus leave the solution, turning the remaining hydrogel a color orresult in the hydrogel becoming cloudy or opaque.

The impedance of the electrode is directly related to the water level inthe hydrogel. As water is lost from the hydrogel the impedance of theelectrode increases. Thus, the precipitate that forms in the gel aswater is lost, is an indirect indicator of an increase in the impedanceof the electrode, and the change in clarity of the hydrogel due to coloror opacity may be utilized as an indication that the electrodepossessing the hydrogel should be changed.

In addition, in alternate embodiments, upon becoming opaque, a messageor symbol that had been visible indicating the electrode was suitablefor use could become obscured, thereby indicating it is time to replacethe electrode. Any symbol that would indicate an electrode was suitablefor use could be utilized. Non-limiting examples of such symbols include“G”, “OK”, “Use”, a smiley face, combinations thereof, and the like. Asthe symbol becomes obscured by the change in opacity due to theprecipitation of the component with limited solubility, an end-userwould be provided with an indication to change the electrode.

Suitable components with limited solubility which may be utilized toindicate the need to change an electrode possessing a hydrogel include,but are not limited to, lithium oxide, boric acid, lithium carbonate,sodium tartrate dibasic dehydrate, sodium hydrogen tartrate,combinations thereof, and the like.

The component with limited solubility may be added in suitable amountsof from about 0.5% by weight of the hydrogel to about 2.5% by weight ofthe hydrogel, in embodiments from about 0.75% by weight of the hydrogelto about 2% by weight of the hydrogel.

Instead of a salt having limited solubility, a thermochromic materialmay similarly be added to the hydrogel as a change indicating component.As noted above, as an electrode is utilized, and water is lost withrepeated use, the impedance of the electrode will increase, therebygenerating heat. The use of a thermochromic material, that is, amaterial which will change its color or opacity on exposure to heat, maybe utilized as an indicator to the user that a temperature increase dueto loss of impedance may be high enough to cause a thermal burn and thusthe electrode should be changed.

Suitable thermochromic materials should undergo a change in color oropacity at a temperature somewhere between body temperature and atemperature at which a first degree thermal burn may occur. Suitablethermochromic materials include thermochromic pigments. Such pigmentsmay include an electron-donating chromogenic substance, anelectron-accepting substance, and a solvent which undergoes a reversiblecolor change in response to a change in temperature. The color changewhich takes place may be from one color to another, from colored tocolorless, or from colorless to colored. Thermochromic materials may becombined so that several temperatures in a predetermined range can beindicated by different colors.

Examples of such thermochromic materials include, but are not limitedto, liquid crystals such as cholesteryl nonanoate, cyanobiphenyls,combinations thereof, and the like. Where utilized, a thermochromicmaterial may be present in an amount from about 0.1% by weight of thehydrogel to about 10% by weight of the hydrogel, in embodiments fromabout 0.5% by weight of the hydrogel to about 5% by weight of thehydrogel.

In other embodiments, an ionic component not soluble in water or havingvery low solubility in water may be added to the hydrogel. The ioniccomponent is added to the hydrogel to saturate the hydrogel, i.e., pastits saturation point, thereby providing the hydrogel with either a coloror opacity. As the electrode is used, the ionic component may beiontophoretically delivered to the patient, for example to the skin towhich the electrode is attached. Thus, after repeated use, with moredelivery of the ionic component, the concentration of the ioniccomponent in the hydrogel will decrease and the hydrogel will lose itscolor or opacity and become clear. In this embodiment, the hydrogelbecoming clear may be an indication that the electrode possessing thehydrogel should be changed.

Suitable ionic components which may be utilized with the hydrogelinclude hydrocortisone acetate and the like. The ionic component may beadded in suitable amounts of from about 3.5% by weight of the hydrogelto about 7.5% by weight of the hydrogel, in embodiments from about 4% byweight of the hydrogel to about 6% by weight of the hydrogel.

In addition, in alternate embodiments, upon changing color or opacity,or becoming clear, a message or symbol could become visible indicatingit is time to change or replace the electrode. Any symbol that wouldindicate a time to change the electrode could be utilized. Non-limitingexamples of such symbols include “N”, “NG”, “S”, “Stop”, “End”,“Replace”, a skull and cross bones, a frowning face, combinationsthereof, and the like.

In yet other embodiments, a wax or other heat sensitive component may beapplied to a surface of an electrode. For example, as noted above, as ahydrogel of the present disclosure dries out with repeated use, theimpedance of an electrode utilizing such a hydrogel will increase,thereby generating heat in the hydrogel. A heat sensitive component witha specific melting point somewhere between body temperature and atemperature at which a first degree thermal burn may occur could beapplied to the upper surface of a substrate utilized in forming anelectrode, on the side of the substrate opposite the hydrogel, andlocated over the area where the connector (wire) contacts the hydrogel.This will be the warmest spot on the electrode. The heat sensitivecomponent may either melt or become clear at a predeterminedtemperature, close to but below the temperature where a burn may occur,thereby indicating the time to change an electrode.

In embodiments, as the temperature increases, the wax or other heatsensitive component may melt or become clear, revealing some message orsymbol underneath the wax or heat sensitive component that indicates itis time to replace the electrode. As described above, any symbol thatwould indicate a time to change the electrode could be utilized.Non-limiting examples of such symbols include “N”, “NG”, “S”, “Stop”,“End”, “Replace”, a skull and cross bones, a frowning face, combinationsthereof, and the like.

Suitable heat sensitive components include, but are not limited to,waxes including paraffin waxes or commercially available waxes such asCARBOWAX® polyethylene glycols and methoxy polyethylene glycols,commercially available from the Dow Chemical Company.

Other suitable heat sensitive components which may be applied to asurface of an electrode include thermochromic materials, including anyof the thermochromic materials described above for placement in thehydrogel. As with the wax described above, the thermochromic materialmay be applied to the surface of the substrate opposite the hydrogelwhere the connector (wire) contacts the hydrogel. As described above,the thermochromic material will change its color or opacity on exposureto heat, thereby indicating to the user that a temperature increase dueto loss of impedance may be high enough to cause a thermal burn.Suitable thermochromic materials include the thermochromic materials andpigments described above for inclusion in the hydrogel itself.Thermochromic materials may be combined so that several temperatures ina predetermined range can be indicated by different colors.

Medical Electrodes

Medical electrodes transmit electrical signals or currents to or from apatient's skin and an external medical apparatus. Medical electrodes arewithin the purview of those skilled in the art. These electrodes mayinclude a conductive composition such as a hydrogel of the presentdisclosure on a substrate. The layer of conductive composition can beadhered to or contacted with the skin of the patient. The medicalelectrode may also include a conductive interface that is electricallyconnected to the layer of conductive composition and adapted to beelectrically connected to an item of external medical equipment. Formany applications, the conductive composition should be sufficientlyadhesive to adhere to the patient's skin, i.e., be a conductiveadhesive. The configuration of the electrode and the adhesive propertiesrequired will depend on the intended application, such as whether theelectrode is a transmission electrode, i.e., an electrode that sendselectric currents or signals to the patient's body, or a sensing ormonitoring electrode, i.e., an electrode that sends electrical signalsfrom the patient's body to external medical equipment.

FIG. 1 and FIG. 2 show a medical electrode 10 on release liner 12.Release liner 12 is a release paper or film of a waxed or coatedplastic, such as a silicone coated polyethylene terephthalate film,which may be used to protect medical electrode 10 before application ofthe electrode to a skin surface.

Electrode 10 includes a layer of a hydrogel of the present disclosure asconductive composition 14. Electrode 10 also includes conductiveinterface 16 having a conductor member with a conductive portion 18contacting layer of conductive composition 14 and tab portion 20extending beyond layer of conductive composition 14 for mechanical andelectrical contact with external medical equipment, such as aelectrocardiogram monitoring (ECG) machine, an electroencephalogram(EEG) machine, or a transcutaneous electrical nerve stimulation (TENS)machine (not shown). Conductive interface 16 includes conductive layer24, coated on at least side 22 of conductive interface 16. Conductivelayer 26 contacts layer of conductive composition 14. Medical electrode10 can be used either as a diagnostic electrocardiogram (ECG or EKG)electrode or as a transcutaneous electrical nerve stimulation (TENS). Inuse, release liner 12, if present, is removed. The layer of conductivecomposition 14 of electrode 10 is applied to the surface of thepatient's skin and electrically connected to the external medicalequipment.

FIG. 3 shows a cross-section of snap medical electrode 30 on a releaseliner. Electrode 30 has nonconductive backing 32, which has opening 33covered by snap 34 through which eyelet 35 protrudes. Snap 34 is securedto eyelet 35. Together snap 34 and eyelet 35 provide at least part of aconductive interface adapted to provide an electrical connection betweena flexible conductive layer 36 and the external medical equipment (notshown). Eyelet 34 and backing 32 are covered by flexible conductivelayer 36 which, in embodiments, may be made of a material such as carbonvinyl. A hydrogel of the present disclosure may be utilized as aconductive composition 40 and adhered to conductive layer 36. Releaseliner 38 protects the conductive composition 40 prior to use. Inembodiments, a complete or partial layer of silver and/or a silver saltsuch as silver chloride may be placed between conductive composition 40and conductive layer 36 (not shown). In embodiments, the layer of silverand/or silver salt may be applied as a coating on at least a portion ofa surface of conductive layer 124. In embodiments, the electrode mayinclude silver (Ag) or silver/silver-chloride (Ag/AgCl) disposed on atleast a portion of the first and/or second sides of the conductivelayer.

As noted above, in embodiments an electrode may have a message or symbolthat either becomes visible or obscured after use, indicating it is timeto replace the electrode. Such a message or symbol, in embodiments, maybe placed on the surface of conductive layer 36 adjacent hydrogel 40(not shown).

In addition, in embodiments, conductive layer 36 and non-conductivelayer 32 may possess contiguous windows adjacent each other in eachlayer (not shown) permitting the visualization of conductive composition40 during use so that changes in color, opacity, and the like may beobserved with an electrode in place on a patient.

In other embodiments, as described above, a heat sensitive component,such as a wax or a thermochromic material, may be applied over eyelet 34(not shown) or a portion of non-conductive backing 32 adjacent eyelet 34(not shown). The heat sensitive component may melt, become clear, orchange color after repeated use and provide an indication of when anelectrode should be changed.

FIG. 4 shows a cross-section of an alternate medical electrode 100 on arelease liner 112, which may be a polyester film or any other materialsuitable for use as a release liner. Electrode 100 includes a layer of ahydrogel of the present disclosure, optionally possessing a pH indicatorand/or other change indicating component as described above, asconductive composition 114. In embodiments, conductive composition 114may have a reinforcement member 113 embedded in the hydrogel, which maybe a woven or a non-woven mesh or any other material, such as a scrim,suitable for forming a reinforcement member. Electrode 100 may alsopossess a conductive layer 124, which may, in embodiments, be a suitablematerial such as a conductive carbon film of suitable thickness, inembodiments about 2 mil In some embodiments, a flood coat or a partialcoating of silver ink 115 (which can be silver and/or silver chloride)may be between conductive layer 124 and conductive composition 114,applied as a coating on at least a portion of a surface of conductivelayer 124. In embodiments, the electrode may include silver (Ag) orsilver/silver-chloride (Ag/AgCl) disposed on at least a portion of thefirst and/or second sides of the conductive layer.

Electrode 100 may also possess a standard stainless steel or tin/copperpig tail lead wire 160 of a suitable length, in embodiments from about 5to about 15 inches long, in other embodiments about 9 inches long. Leadwire 160 may possess an insulation jacket which, in turn, may be boundto conductive layer 124 using an adhesive 170. Electrode 100 may alsopossess a reinforcement film 150 having a medical grade pressuresensitive adhesive (PSA) thereon overlying lead wire 160 and affixingreinforcement film 150 to both conductive layer 124 and cover material180. Finally, cover material 180 may possess pull tab 190 notched out ofcover material 180 on the end of electrode 100 opposite the end intowhich the lead wire 160 enters the electrode.

FIG. 5 is a top view of an electrode of the present disclosure prior touse. From the top, flexible conductive layer 36 and overlying releaseliner 38, are visible. Eyelet 34 is also visible in the center offlexible conductive layer 36. FIG. 6 is a top view of an electrode ofthe present disclosure after use, and thus does not depict release liner38. As can be seen in FIG. 6, a message or symbol has become visible onconductive layer 36 after use, in this case the word “STOP”, indicatingit is time to replace the electrode.

Medical electrodes may be packaged for use in any suitable materialswithin the purview of those skilled in the art. For example, electrodesmay be packaged in materials such as polyethylene or other plasticfilms, foil barrier packaging, combinations thereof, and the like.

INDUSTRIAL APPLICABILITY

The conductive compositions of the present disclosure may be useful withmedical electrodes that can be used with medical equipment for a varietyapplications, such as: electrocardiogram monitoring (ECG) electrodes(tab and snap style) for monitoring heart activity and for diagnosingheart abnormalities; electroencephalogram (EEG) electrodes;transcutaneous electrical nerve stimulation (TENS) electrodes used forpain management; neuromuscular stimulation (NMS) used for treatingconditions such as scoliosis; muscle stimulation electrodes; woundtreatment electrodes (accelerated healing of skin wounds or brokenbones); defibrillation electrodes to dispense electrical energy to achest cavity of a mammalian patient to defibrillate heart beats of thepatient; iontophoresis; and dispersive electrodes to receive electricalenergy dispensed into an incision made during electrosurgery. Otherapplications of the conductive compositions of the invention include,for example, electro-surgical dispersive pads; drug delivery (passive oriontophoretic); pre-surgical limb or area markers, tapes (anchoringchest tubes, NG tubes, IVs, cannulae, etc); and sterile seals at needleor cannula entry points. The medical equipment used in theseapplications is within the purview of those skilled in the art.

Example 1

A hydrogel sample was prepared containing phenolphthalein as a pHindicator. The purpose was to see if a change in pH of the hydrogel dueto hydrolysis was significant enough to be observed over the useduration. About 49.5 grams of RG-63B monomer solution (lot #072686, madeSep. 25, 2007) was combined with about 0.5 grams of a 1% phenolphthaleinsolution in alcohol.

The monomer solution was first weighed into a clean glass 200 ml beaker.The phenolphthalein solution was added to it and the mix placed under anelectric mixer with a prop style blade and mixed for about 5 minutespulling the vortex half way down the mix shaft. Mixing was stopped andthe shaft cleaned. A 25 mil thick film of the monomer was cast on anon-woven polyester scrim on a 5 mil polyester release liner. The filmwas irradiated for about 30 seconds under a Xenon arc UV lamp. Theresulting polymer film was covered with a 2.5-mil polyethylene releaseliner, pouched and sealed in a poly-foil bag. TENS electrodes were madeincluding this hydrogel. The hydrogel sides of the electrodes wereplaced on a conductive gel, simulating skin and tissue. They wereattached to a commercial TENS device and electrical stimulation wasinitiated. After about four hours of stimulation, the hydrogel on thecathode side had turned fuchsia, indicating the pH of the hydrogel was 8or higher.

It will be appreciated that various of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims Unless specifically recited in aclaim, steps or components of claims should not be implied or importedfrom the specification or any other claims as to any particular order,number, position, size, or material.

1. A medical electrode comprising: a substrate; and a conductivecomposition on at least a portion of a surface of the substrate, theconductive composition comprising at least one hydrogel and at least onecomponent having a limited solubility in water that precipitates fromthe hydrogel after repeated use of the electrode, thereby providing anindication to replace the electrode.
 2. The medical electrode of claim1, wherein the hydrogel comprises a component selected from the groupconsisting of gelatin, polysaccharides, crosslinked acrylamide polymers,hydroxyethylmethacrylate polymers, crosslinked polyhydroxyethylacrylate,polymerized, crosslinked 2-acrylamido-2-methylpropane sulfonic acidpolymers, crosslinked polyvinylpyrrolidone, polyacrylic acid, copolymersof the foregoing, one or more salts thereof, and combinations thereof.3. The medical electrode of claim 1, wherein the hydrogel comprises acopolymer comprising a first monomer comprising a mixture of acrylicacid and a salt thereof, present in an amount of from about 8 weight %to about 85 weight % of the hydrogel, and a second monomer of theformula CH₂═CHC(O)XR, in which X is O or NH and R is an unsubstituted orsubstituted alkyl group of from about 1 to about 5 carbon atoms presentin an amount of from about 15 weight % to about 92 weight % of thehydrogel.
 4. The medical electrode of claim 1, wherein the hydrogelfurther comprises an electrolyte present in an amount of from about 0.5%by weight to about 10% by weight of the hydrogel, and optionally aneutralizer selected from the group consisting of ammonium hydroxide,sodium hydroxide, potassium hydroxide, lithium hydroxide, andcombinations thereof, optionally further comprises a cross linking agentselected from the group consisting of N—N′-methylene bis-acrylamide,diethylene glycol diacrylate, diethylene glycol dimethacrylate,trimethylolpropane triacrylate, trimethylolpropane trimethacrylate,ethoxylated trimethylolpropane triacrylate, ethoxylatedtrimethylolpropane trimethacrylate, pentaerythritol triacrylate,pentaerythritol trimethacrylate, pentaerythritol tetraacrylate,pentaerythritol tetramethacrylate, polyethylene glycol diacrylate,polyethylene glycol dimethacrylate, and combinations thereof, andoptionally further comprises a polymerization initiator selected fromthe group consisting of 2,2-azobisisobutyronitrile,1-hydroxycyclohexylphenyl ketone,2-hydroxy-2-methyl-1-phenylpropan-1-one,2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propan-1-one,2,2-dimethoxy-2-phenylacetophenone, benzophenone, and combinationsthereof.
 5. The medical electrode of claim 1, wherein the at least onecomponent that precipitates from the hydrogel is selected from the groupconsisting of lithium oxide, boric acid, lithium carbonate, sodiumtartrate dibasic dehydrate, sodium hydrogen tartrate, and combinationsthereof, present in an amount of from about 0.5% by weight of thehydrogel to about 2.5% by weight of the hydrogel.
 6. The medicalelectrode of claim 1, wherein the indication to replace the electrodecomprises a symbol or message, and wherein the electrode optionallyfurther comprises a conductive layer possessing a coating of silver orsilver/silver-chloride on at least a portion of a surface of theconductive layer.
 7. A medical electrode comprising: a substrate; and aconductive composition on at least a portion of a surface of thesubstrate, the conductive composition comprising at least one hydrogeland at least one ionic component that may be iontophoretically deliveredfrom the hydrogel after repeated use of the electrode, thereby providingan indication to replace the electrode.
 8. The medical electrode ofclaim 7, wherein the hydrogel comprises a component selected from thegroup consisting of gelatin, polysaccharides, crosslinked acrylamidepolymers, hydroxyethylmethacrylate polymers, crosslinkedpolyhydroxyethylacrylate, polymerized, crosslinked2-acrylamido-2-methylpropane sulfonic acid polymers, crosslinkedpolyvinylpyrrolidone, polyacrylic acid, copolymers of the foregoing, oneor more salts thereof, and combinations thereof.
 9. The medicalelectrode of claim 7, wherein the hydrogel comprises a copolymercomprising a first monomer comprising a mixture of acrylic acid and asalt thereof, present in an amount of from about 8 weight % to about 85weight % of the hydrogel, and a second monomer of the formulaCH₂═CHC(O)XR, in which X is O or NH and R is an unsubstituted orsubstituted alkyl group of from about 1 to about 5 carbon atoms presentin an amount of from about 15 weight % to about 92 weight % of thehydrogel.
 10. The medical electrode of claim 7, wherein the hydrogelfurther comprises an electrolyte present in an amount of from about 0.5%by weight to about 10% by weight of the hydrogel, and optionally aneutralizer selected from the group consisting of ammonium hydroxide,sodium hydroxide, potassium hydroxide, lithium hydroxide, andcombinations thereof, optionally further comprises a cross linking agentselected from the group consisting of N—N′-methylene bis-acrylamide,diethylene glycol diacrylate, diethylene glycol dimethacrylate,trimethylolpropane triacrylate, trimethylolpropane trimethacrylate,ethoxylated trimethylolpropane triacrylate, ethoxylatedtrimethylolpropane trimethacrylate, pentaerythritol triacrylate,pentaerythritol trimethacrylate, pentaerythritol tetraacrylate,pentaerythritol tetramethacrylate, polyethylene glycol diacrylate,polyethylene glycol dimethacrylate, and combinations thereof, andoptionally further comprises a polymerization initiator selected fromthe group consisting of 2,2-azobisisobutyronitrile,1-hydroxycyclohexylphenyl ketone,2-hydroxy-2-methyl-1-phenylpropan-1-one,2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propan-1-one,2,2-dimethoxy-2-phenylacetophenone, benzophenone, and combinationsthereof.
 11. The medical electrode of claim 7, wherein the at least oneionic component that may be iontophoretically delivered from thehydrogel comprises hydrocortisone acetate, present in an amount of fromabout 3.5% by weight of the hydrogel to about 7.5% by weight of thehydrogel.
 12. The medical electrode of claim 7, wherein the indicationto replace the electrode comprises a symbol or message, and wherein theelectrode optionally further comprises a conductive layer possessing acoating of silver or silver/silver-chloride on at least a portion of asurface of the conductive layer.
 13. A medical electrode comprising: asubstrate; a conductive composition on at least a portion of a surfaceof the substrate, the conductive composition comprising at least onehydrogel; and a heat sensitive component on a portion of a surface ofthe substrate opposite the surface of the substrate having theconductive composition, wherein the heat sensitive component provides anindication to replace the electrode on exposure to heat.
 14. The medicalelectrode of claim 13, wherein the hydrogel comprises a componentselected from the group consisting of gelatin, polysaccharides,crosslinked acrylamide polymers, hydroxyethylmethacrylate polymers,crosslinked polyhydroxyethylacrylate, polymerized, crosslinked2-acrylamido-2-methylpropane sulfonic acid polymers, crosslinkedpolyvinylpyrrolidone, polyacrylic acid, copolymers of the foregoing, oneor more salts thereof, and combinations thereof.
 15. The medicalelectrode of claim 13, wherein the hydrogel comprises a copolymercomprising a first monomer comprising a mixture of acrylic acid and asalt thereof, present in an amount of from about 8 weight % to about 85weight % of the hydrogel, and a second monomer of the formulaCH₂═CHC(O)XR, in which X is O or NH and R is an unsubstituted orsubstituted alkyl group of from about 1 to about 5 carbon atoms presentin an amount of from about 15 weight % to about 92 weight % of thehydrogel.
 16. The medical electrode of claim 13, wherein the hydrogelfurther comprises an electrolyte present in an amount of from about 0.5%by weight to about 10% by weight of the hydrogel, and optionally aneutralizer selected from the group consisting of ammonium hydroxide,sodium hydroxide, potassium hydroxide, lithium hydroxide, andcombinations thereof, optionally further comprises a cross linking agentselected from the group consisting of N—N′-methylene bis-acrylamide,diethylene glycol diacrylate, diethylene glycol dimethacrylate,trimethylolpropane triacrylate, trimethylolpropane trimethacrylate,ethoxylated trimethylolpropane triacrylate, ethoxylatedtrimethylolpropane trimethacrylate, pentaerythritol triacrylate,pentaerythritol trimethacrylate, pentaerythritol tetraacrylate,pentaerythritol tetramethacrylate, polyethylene glycol diacrylate,polyethylene glycol dimethacrylate, and combinations thereof, andoptionally further comprises a polymerization initiator selected fromthe group consisting of 2,2-azobisisobutyronitrile,1-hydroxycyclohexylphenyl ketone,2-hydroxy-2-methyl-1-phenylpropan-1-one,2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propan-1-one,2,2-dimethoxy-2-phenylacetophenone, benzophenone, and combinationsthereof.
 17. The medical electrode of claim 13, wherein the heatsensitive component is selected from the group consisting of paraffinwaxes, polyethylene glycols, and methoxy polyethylene glycols.
 18. Themedical electrode of claim 13, wherein the heat sensitive componentcomprises a thermochromic material comprising a liquid crystal selectedfrom the group consisting of cholesteryl nonanoate, cyanobiphenyls, andcombinations thereof.
 19. The medical electrode of claim 13, wherein theindication to replace the electrode comprises a symbol or message, andwherein the electrode optionally further comprises a conductive layerpossessing a coating of silver or silver/silver-chloride on at least aportion of a surface of the conductive layer.